Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

Abstract

This review focuses on recent progress made using well-defined molecular chromium complexes that, upon suitable activation, can catalyze the tri-, tetra, oligo-and/or polymerization of ethylene. In particular, emphasis will be placed on the tuning of the performance characteristics of these homogeneous catalysts through structural modifications made to the multidentate ligand manifold (e.g., donor atoms, charge, backbone and strain) and the effects these changes have on the resulting ethylene derivatives. While the ability of these catalysts to mediate the formation of high molecular weight linear polyethylene continues to see many developments, their capacity to form polyethylene waxes and oligomers has witnessed some major advances. Moreover, the impressive selectivity of some chromium systems to generate commercially important 1-hexene and more recently 1-octene has seen the implementation of this technology at the industrial level. The types of precatalysts to be discussed will be divided broadly on the basis of their ability to generate either polymers/oligomers or short chain αolefins; the effects of co-catalyst and reaction conditions (e.g., temperature, pressure, solvent) on catalytic activity and selectivity, will be also developed. In addition, current proposals as to the mechanistic details displayed by these versatile chromium catalysts will be highlighted.

NNNO‐Heteroscorpionate nickel (II) and cobalt (II) complexes for ethylene oligomerization: the unprecedented formation of odd carbon number olefins

“…[1] The catalytic properties of corresponding chromium-based complexes, which selectively trimerize ethylene to 1-hexene, have been studied in sufficient detail. [2][3][4] The use of heteroscorpionate complexes of transition, [5,6] sblock [7,8] and p-block [9,10] metals as catalysts for lactide and ε-caprolactone ring-opening polymerization has also attracted scientific attention in the last decade. At the same time, the heteroscorpionate complexes of the iron triad metals, unlike other N-donor ligands, [11,12] have been studied very superficially in the olefin coupling catalysis.…”

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

“…[ 9–12 ] Among all the transition‐metal‐based catalysts, chromium catalysts have proven to be the most promising candidates for selective ethylene oligomerization. [ 13–16 ] In this way, significant efforts have been dedicated to the synthesis of new families of ligands based on a wide variety of donor‐group combinations aiming to generate more efficient chromium catalyst systems that are capable of selectively forming α‐olefins with high productivities. [ 17–81 ] In particular, chromium catalysts stabilized by pyrazolyl‐based tridentate ligands have been successfully synthetized and their catalytic application in ethylene oligomerization explored in details.…”

Extremely Active Ethylene Tetramerization Catalyst Avoiding the Use of Methylaluminoxane: [iPrN{P(C6H4‐p‐SiR3)2}2CrCl2]+[B(C6F5)4]−

“…Ethylene oligomerization catalysts typically generate 1‐alkenes with an even number of carbon atoms such as 1‐hexene, 1‐octene, 1‐decene, etc., which are also called whole linear α‐olefins (LAOs) . 1‐Hexene and 1‐octene are used as comonomers in the ethylene polymerization processes, and their demand has increased with the increase in the production capacity of linear low‐density polyethylene (LLDPE) and polyolefin elastomer (POE).…”

“…Ethylene oligomerization catalysts typically generate 1-alkenes with an even number of carbon atoms such as 1-hexene, 1octene, 1-decene, etc., which are also called whole linear αolefins (LAOs). [1][2][3][4][5] 1-Hexene and 1-octene are used as comonomers in the ethylene polymerization processes, and their demand has increased with the increase in the production capacity of linear low-density polyethylene (LLDPE) and polyolefin elastomer (POE). Whereas nickel-and zirconium-based catalysts are used in the Shell higher-olefin process and Sabic/ Linde Alpha-SABLIN process, respectively, to produce a wide range of LAOs (Flory-Schulz distribution), [6,7] Cr-based catalysts that can selectively generate 1-hexene and/or 1-octene have also been discovered.…”